Analog frequency modulation (FM) broadcast band receivers can be impaired by noise, multipath and interference from blocker signals. These impairments will often show up as static in the stereo audio output for the tuned analog FM channel. Analog frequency demodulated audio signals degrade gradually with noise and channel impairments. Therefore analog FM receivers apply gradual mitigation techniques such as stereo blend (removing stereo content) and hi-cut (attenuating high frequency audio components) to the demodulated audio signal. The amount of mitigation is gradual and based on received signal quality metrics such as signal-to-noise ratio (SNR), received signal strength indicator (RSSI) and multipath indicator. SNR and multipath metrics can be computed, for example, by analyzing the amplitude modulation in the received FM signal.
FIG. 1 illustrates an example of a conventional stereo blending relationship for the demodulated audio signal of an analog FM receiver, showing percentage stereo as a function of radio frequency (RF) SNR. In FIG. 1, full stereo (100% stereo) is output at the RF SNR value corresponding to point b, and full mono (or 0% stereo) is output at the RF SNR value corresponding to point a, with percentage stereo output varying from 0% to 100% at SNR values between point a and b according to the relationship shown. FIG. 2 illustrates an example of conventional hi-cut filter attenuation (demodulated audio corner frequency) as a function of RF SNR for a conventional FM receiver that results in a gradual drop in high frequency audio signal components with decreasing SNR. In FIG. 2, full audio signal frequency bandwidth is produced at the SNR value corresponding to point d (corner frequency maximum), while a reduced minimum audio signal frequency bandwidth is produced at the SNR value corresponding to point c (corner frequency minimum). The amount of high frequency components decreases between the SNR values of point d and c according to the relationship shown. For the illustrated case of FIGS. 1 and 2, the SNR values corresponding to points a, b, c and d that define the region over which mitigation occurs are programmable at circuit design time. Information on receiver signal processing and signal mitigation techniques may be found in U.S. Pat. No. 7,272,375; U.S. Pat. No. 8,023,918; U.S. Pat. No. 8,358,994; and U.S. Pat. No. 8,417,206, each of which is incorporated herein by reference in its entirety.
Digital radios exist that enable reception of digital radio signals that provide improved fidelity over analog radio signals, as well as additional features. Currently in the United States, digital radio is available over-the-air using sidebands to an analog carrier signal. The current system as commercialized in the United States is referred to as so-called HD™ radio or “HD Radio.” By way of these sidebands, a broadcaster can provide one or more additional complementary channels to an analog signal. Accordingly, digital or HD™ radios can receive these signals and digitally demodulate them to provide a higher quality audio signal that includes the same content as an analog radio signal, or to provide additional content to the analog radio signal such as supplementary broadcasting available on one or more supplemental digital channels. Typically, a digital radio tuner is incorporated into a HD™ radio solution that also includes a conventional analog FM receiver for handling analog demodulation of a corresponding simulcast FM analog broadcast signal that includes the same information (audio program content) as the HD™ digital broadcast signal. Where audio content of the selected digital demodulated channel is the same as the selected analog demodulated channel, blending from the digital demodulated channel to the analog demodulated channel may occur to resolve situations in which the digital channel is temporarily lost.
In contrast to the gradual degradation of demodulated analog audio signals, digital demodulated (HD™ radio) audio signals degrade abruptly from full audio fidelity to noise over a short range of RF SNR level. Moreover digital radio signals require a higher RF SNR level for demodulation than analog radio signals. As shown in FIG. 3A, audio fidelity of digital demodulated HD™ radio signals abruptly drops from maximum audio signal fidelity (full fidelity signal that is full-stereo and with no hi-cut applied) to all noise (0% fidelity) at a RF SNR level corresponding to loss of the digital signal. Accordingly, HD™ radio systems switch the audio output from the digital demodulated HD™ digital audio signal to the analog demodulated FM analog audio signal when the received RF signal level drops below a selected SNR threshold slightly above the level corresponding to loss of digital audio. Conversely, when the RF SNR again increases above a selected SNR threshold, the reverse operation occurs, i.e., the audio output is switched from the analog demodulated audio signal to the digital demodulated audio signal.
The switching event between demodulated digital audio signal to demodulated analog audio signal (and vice-versa) is commonly referred to as the in-band on-channel (IBOC) blend. This IBOC blend operation is a cross-fade operation over time (typically a few seconds) between the two audio sources, and is under control of the HD demodulator, which produces a 1-bit blend control signal (blend flag) that triggers the blend operation as shown in FIG. 3B. In this regard, a 0-to-1 transition of the blend flag triggers a crossfade into digital, a 1-to-0 transition triggers a crossfade into analog. The blending threshold from demodulated digital audio signal to demodulated analog audio signal and the blending threshold from demodulated analog audio signal to demodulated digital audio signal are set above the loss of digital audio point and there is typically some hysteresis to these thresholds In a mobile receiver environment, the IBOC blend can occur often, causing the user to experience abrupt changes in audio fidelity between full-fidelity digital audio and partial fidelity FM audio (e.g., mono blended or hi-cut FM audio) over some RF signal levels. Under such conditions, some users may use radio settings to disable high definition reception.
FIG. 4 illustrates a block diagram of a conventional digital FM radio receiver system 400 that includes analog receiver circuitry. As shown, system 400 includes an antenna 402 that is coupled to RF front end circuitry 404, which includes a mixer to downconvert incoming RF signals to a lower frequency. The output of RF front end circuitry 404 is provided to analog-to-digital conversion (ADC) circuitry 406, which provides a digitized signal output to digital front end circuitry 408 that performs tasks such as channelization and filtering. Digital front end circuitry in turn provides the same radio channel information as processed output signals to an analog demodulation path that includes FM discriminator (demodulation) circuitry 410 and to a digital demodulation path that includes HD™ demodulation circuitry 430 as shown. FM discriminator circuitry 410 in turn provides analog demodulated (multiplex) signals to FM multiplex (MPX) decoder circuitry 412 that in turn produces separate L+R (left plus right) and L−R (left minus right) signals as shown. HD™ demodulation circuitry 430 digital demodulates the processed digital information from digital front end circuitry 408 and provides a HD™ demodulated L (left) and R (right) signals according to an I2S protocol. Also shown coupled to the input of the FM discriminator is signal metrics circuitry 420 that measures signal quality metrics on the modulated FM signal such as SNR, RSSI, and multipath. FM signal metrics circuitry 422 is coupled to the output of the FM discriminator and measures signal quality metrics on the FM demodulated signals such as audio SNR and DC offset.
Still referring to FIG. 4, the separate demodulated L+R and L−R signals from FM multiplex (MPX) decoder circuitry 412 are next provided to signal quality mitigation components 450 that include hi-cut circuitry 414 and stereo blend circuitry 416. The separate L+R and L−R signals are processed in block 450 by hi-cut circuitry 414 that varies the audio frequency bandwidth according to the varying signal quality metrics received from metrics circuitry 420 using a frequency bandwidth control relationship such as described in relation to FIG. 2. The separate L+R and L−R signals are then blended together between full stereo and full mono FM audio output by stereo blend circuitry 416 according to varying signal quality metrics received from metrics circuitries 420 and 422 using a stereo blending relationship such as described in relation to FIG. 1. The resulting mitigated demodulated FM audio signal including left and right audio signals is then provided to IBOC blend circuitry 418 from signal quality mitigation circuitry components 450. IBOC blend circuitry 418 includes a cross-fader that blends between demodulated FM stereo audio signal output and HD™ demodulation circuitry 430 audio output according to a blend control signal received from HD™ demodulation circuitry 430 as shown. The blend control signal from HD™ demodulation circuitry 430 controls blend circuitry 418 to implement the IBOC blend cross-fade operation described in relation to FIG. 3B in response to varying SNR of processed digital signal from digital front end 408. Using the conventional system architecture of FIG. 1, demodulated FM audio signals (left and right audio) that are potentially mitigated by stereo blend and/or hi-cut operations described above are then provided to IBOC blend circuitry 418 along with full fidelity HD™ audio signals from HD™ demodulation circuitry 430 such that IBOC blend circuitry cross fades between two audio signals (demodulated FM analog broadcast and demodulated HD™ digital broadcast) that differ in audio fidelity under certain received RF signal conditions. Information regarding digital radio receiver processing and blending techniques for digital and analog signals may be found in United States Patent Publication No. 2012/0082271; United States Patent Publication No. 2012/0108191; and in U.S. Pat. No. 8,195,115 each of which is incorporated herein by reference in its entirety.